The age and formation of the Scandinavian mountains and the Norwegian strandflat have long been the subject of dispute. Some researchers argue that the present-day mountains are remains of the Caledonian orogen while others claim that the Caledonian nappes after erosion were buried by Mesozoic sediments and subsequently exhumed. In order to clarify these issues, we have studied remains of chemically weathered rocks (saprolites) from the coast to the interior of central Norway. The multidisciplinary study includes digital topography, electrical resistivity tomography (ERT), XRD, XRF, palynological analyses and K–Ar dating of samples from outcrops, trenches and core drilling. The coastal areas are dominated by an outer strandflat and an inner joint-valley landscape, while the interior is characterised by smoother landscapes referred to as palaeo-surfaces. Remnants of pre–Tertiary weathering occur in the joint valley landscape as well as on the palaeo-surfaces. The saprolites are found within fault- and fracture-zones and at depths exceeding 50 m in drillholes. It is suggested that the old saprolites were strongly eroded along the coast and in the fjords and valleys such as in Orkdalen and Sunndalen. K–Ar dating of mainland clay alteration most frequently yielded Jurassic ages along a profile extending from the coast to the Dovrefjell mountains (c. 1400 m a.s.l.). The formation age of the smectite- and kaolinite-containing saprolites seems to be almost contemporaneous along this profile implying that the entire area was subject to weathering in a warm and humid climate, such as prevailed during the Late Triassic and Jurassic. Palynological residues containing thermally altered Triassic and Jurassic pollen and spores in the clay-infected bedrock lend support to the saprolite interpretation. The Mesozoic landscape in central Norway was consequently shaped by uplift and deep weathering in the Jurassic. The entire Trøndelag county was most likely covered by Mesozoic sedimentary rocks until Cenozoic exhumation. The landscape was modified by Cenozoic tectonic uplift and erosion, and finally reworked by Pleistocene glacial erosion. We therefore conclude that both the observed saprolites and the shape of the present-day landscape in central Norway give a strong impression of the original Jurassic weathering surface.
Summary There was a quick-clay landslide in Byneset on 1st January 2012. The landslide area is surrounded mostly by agricultural lands. The geology in the area consists of old ocean floor with outcropped bedrocks at several places. Prehistorically, the sea-level was ∼160 m higher than the present sea-level. The sediment in the area mainly consists of marine clay. Subsequent leaching by fresh groundwater alters the chemical composition of the pore water and “quick clay” may develop. Quick clay completely liquefies when remolded under stress and results in large landslides. Resistivity survey is a powerful tool to investigate such clay layers but it is time consuming. Airborne EM survey can be a faster way to investigate large areas. Seismic refraction survey is useful to delineate depth of bedrock. Therefore frequency domain helicopter EM (FHEM), resistivity and refraction seismic surveys were performed in the area last year. There is a good agreement of the results from all the geophysical surveys. Resistivity and FHEM data show various clay layer boundaries well in case of thin marine clay deposits. However, FHEM data is poor to resolve geological boundaries in case of thicker marine clay deposits due to low skin depth in a conductive environment.
There was a quick-clay landslide in Byneset, Mid Norway on 1st January 2012. The landslide area is surrounded mostly by agricultural lands. The geology in the area consists of old ocean floor with outcropped bedrock at several places. Prehistorically, the sea-level was ~160 m higher than the present sea level. Resistivity survey is a powerful tool to investigate marine sediments but it is relatively time consuming. Airborne EM survey is a faster way to investigate large areas. Therefore, Airborne and ground geophysical surveys were performed in the study area in Autumn 2013. Main aim for these surveys was to see usefulness of Frequency-domain Helicopter-borne ElectroMagnetic (FHEM) data in mapping of the marine sediments and cross-check it with 2D resistivity, refraction seismic and other available data like geotechnical investigations including RCPTu. Interpretation of FHEM data showed some correlation with 2D resistivity and refraction seismic data. Comparison of FHEM results with 2D resistivity, refraction seismic results, known bedrock depth from drilling and exposed bedrock locations suggests that FHEM data can be used for clay layer mapping and to indicate a rough bedrock depth. It can differentiate between layers of unleached marine clay (< 10 Ωm) and leached marine clay or possible quick clay (10-100 Ωm). However, similar resistivity values of possible quick clay (10-100 Ωm) can also suggest non-quick, leached clay and silty sediments. FHEM data is poor to resolve geological boundaries in case of thicker marine clay deposits, due to low skin depth in a conductive environment.
The conventional, L2-norm-based, regularization term in electromagnetic (EM) inversions implements smooth constraints on model complexity in the space domain, which can smoothen the boundaries of complex underground structures. To improve the resolution of 3-D frequency-domain airborne EM (AEM) inversions, we propose a new algorithm for sparse-regularized inversion based on the shearlet transform. Unlike traditional methods that invert the model parameters in the space domain, we first transform the 3-D resistivity model into the frequency domain and then invert the sparse coefficients using an L1-norm measure to ensure the sparseness of the solution. Finally, we transform the shearlet coefficients back to the space domain to update the model. The shearlet transform has inherent multiscale and multidirectional properties, making it capable of effectively extracting complex geometries such as curved boundaries. We adopt the finite-difference method and the iteratively reweighted least-squares scheme for our 3-D AEM modeling and inversions and apply the "moving footprint" technique to speed up the inversion. Tests using synthetic data show that sparse-regularized inversion based on the shearlet transform can obtain more-focused inversion results than conventional smoothness-constrained inversions based on the L2-norm. Tests using field survey data also reveal that the new method can achieve more realistic underground structures.
Global demand for critical raw materials, including phosphorus (P) and rare earth elements (REEs), is on the rise. The south part of Norway, with a particular focus on the Southern Oslo Rift region, is a promising reservoir of Fe-Ti-P-REE resources associated with magmatic systems. Confronting challenges in mineral exploration within these systems, notably the absence of alteration haloes and distal footprints, we have explored alternative methodologies. In this study, we combine machine learning with geological expertise, aiming to identify prospective areas for critical metal prospecting. Our workflow involves processing over 400 rock samples to create training datasets for mineralization and non-mineralization, employing an intuitive sampling strategy to overcome an imbalanced sample ratio. Additionally, we convert airborne magnetic, radiometric, and topographic maps into machine learning-friendly features, with a keen focus on incorporating domain knowledge into these data preparations. Within a binary classification framework, we evaluate two commonly used classifiers: a random forest (RF) and support vector machine (SVM). Our analysis shows that the RF model outperforms the SVM model. The RF model generates a predictive map, identifying approximately 0.3% of the study area as promising for mineralization. These findings align with legacy data and field visits, supporting the map’s potential to guide future surveys.
ABSTRACT The very low‐frequency (VLF) electromagnetic method utilizes primary signals (field) transmitted from worldwide distant transmitters located in coastal areas. These transmitters are meant for long distance marine communication. VLF transmitters operate at a low communication frequency band (between 5–30 kHz) and the transmitted signal travels a long distance. Transmitted signals penetrate the Earth’s subsurface and produce electromagnetic induction in the subsurface even several thousands of kilometres away from the transmitters. The VLF method is quite simple and frequently used in the delineation of near‐surface conducting structures of various practical applications. Several conducting structures lying along a measured profile with different conductivities can be properly induced at distinct frequencies that yield the maximum response. Therefore, such conductors may not be identified or resolved well using single frequency VLF measurement. A 2D numerical modelling study was carried out over a wide frequency range (1–500 kHz) to find the frequency that produces the maximum response for a given conductor. Results show that a particular frequency (focusing frequency) produces the maximum (peak) response for a conductor. When the measuring frequency either increases or decreases with respect to the focusing frequency, then the peak response always decreases. The focusing frequency remains almost similar with an increase in target depth and host resistivity. An increase in the overburden conductivity shows a decline in the focusing frequency. Two or more targets of different conductivity present in the subsurface yield peak responses at corresponding focusing frequencies. This shows that they will be resolved well at corresponding focusing frequencies. In such circumstances, inversion using single frequency VLF data yields inaccurate results. However, the use of multi‐frequency VLF data yields better results. Inversion of multi‐frequency VLF data is presented to show the efficacy of the approach. A field measurement is also presented and the effectiveness of multi‐frequency VLF measurement is highlighted. Since the numerical modelling study is performed over a broad frequency range covering the VLF and radiomagnetotelluric signal, the focusing study is valid for radiomagnetotelluric applications as well.
Summary The Ramså Basin on Andøya, Northern Norway is the only known Mesozoic basin onshore Norway. The primary basement rocks in the area are granodiorite. The Mesozoic sedimentary strata of the Ramså basin comprise sandstones and different clay stones of Cretaceous and Jurassic ages. Resistivity of the subsurface can be investigated by electric and electromagnetic (EM) geophysical methods to delineate the extension of the Ramså basin. Borehole logging, electrical resistivity tomography (ERT) and helicopter-borne frequency-domain EM (HEM) surveys were performed in the area in the frame of different projects to gain a better understanding of the basin geology and structural settings. A quasi-3D inversion of HEM data shows a good agreement with borehole resistivity and ERT resistivity. HEM interpretation presents a 3D subsurface resistivity image of a larger area in comparison to what could have been drawn from few ERT lines and borehole logging. HEM interpretation together with other ERT and borehole resistivity indicates extension of Ramså basin boundary beyond presently mapped boundary. Two new highly conductive areas are identified by HEM data interpretation and one of the areas is confirmed with presence of graphite and sulfide minerals by geological observations.
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